Known in the past has been an internal combustion engine which provides an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and controls the amount of fuel fed to a combustion chamber of the internal combustion engine based on the output of this air-fuel ratio sensor.
As one example of an air-fuel ratio sensor, there is known an air-fuel ratio sensor which changes in output current linearly (proportionally) to an exhaust air-fuel ratio (for example, PLT 1). The output current becomes larger the higher the exhaust air-fuel ratio (the leaner it becomes). For this reason, by detecting the output current of the air-fuel ratio sensor, it is possible to estimate the exhaust air-fuel ratio.
However, an air-fuel ratio sensor has somewhat of a response delay. If the response delay of the downstream side air-fuel ratio sensor excessively increases, a change in the air-fuel ratio of the exhaust flowing out from the exhaust purification catalyst cannot be quickly detected, so the exhaust emission is liable to deteriorate. Therefore, PLTs 1 and 2 propose an abnormality diagnosis system for diagnosing abnormality of response delay of a downstream side air-fuel ratio sensor.